Recent work has shown that axions can be efficiently produced via nonstationary pair plasma discharges in the polar cap region of pulsars. Here, we point out that for axion masses 10−9≲ma≲10−4eV a sizable fraction of the sourced axion population will be gravitationally confined to the neutron star. These axions accumulate over astrophysical timescales, thereby forming a dense “axion cloud” around the star. We argue that the existence of such a cloud, with densities reaching and potentially exceeding O(1022) GeV cm−3, is a generic expectation across a wide range of parameter space. For axion masses ma≳10−7 eV, energy is primarily radiated from the axion cloud via resonant axion-photon mixing, generating a number of distinctive signatures that include a sharp line in the radio spectrum of each pulsar (located at the axion mass, and with an order percent-level width) and transient events arising from the reconfiguration of charge densities in the magnetosphere. While a deeper understanding of the systematic uncertainties in these systems is required, our current estimates suggest that existing radio telescopes could improve sensitivity to the axion-photon coupling by more than an order of magnitude. Published by the American Physical Society 2024

中文翻译：

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中子星周围的轴子云


最近的研究表明，轴子可以通过脉冲星极帽区域的非平稳对等离子体放电有效地产生。在这里，我们指出，对于质量为 10-9≲马≲10-4eV 的轴子群，相当大一部分来源的轴子群将在引力作用下局限于中子星。这些轴子在天体物理时间尺度上积累，从而在恒星周围形成密集的“轴子云”。我们认为，这种云的存在，其密度达到并可能超过 O（1022） GeV cm-3，是广泛参数空间的一般预期。对于轴子质量 马≳10−7 eV，能量主要通过共振轴子-光子混合从轴子云辐射，产生许多独特的特征，包括每个脉冲星的无线电频谱中的一条尖线（位于轴子质量处，具有数量级百分比级宽度）和由磁层中电荷密度的重新配置引起的瞬态事件。虽然需要更深入地了解这些系统中的系统不确定性，但我们目前的估计表明，现有的射电望远镜可以将对轴子-光子耦合的灵敏度提高一个数量级以上。 美国物理学会 2024 年出版